Hot mold steel for long life cycle die casting having high thermal conductivity and method for preparing the same

ABSTRACT

According to an embodiment, the described hot mold steel may be excellent in high thermal conductivity to decrease the temperature difference in materials at high temperature, thereby making heat-checking properties excellent. When the hot mold steel according to the present disclosure is used for die casting, the cooling rate of the product produced using the die casting is quick, thereby improving the physical properties of the produced product and shortening the cooling time to improve productivity. Furthermore, the hot mold steel may have excellent high temperature durability, such that the die casting produced using the hot mold steel may have characteristics of a long life cycle.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to Korean Patent Application No. 10-2016-0154960, filed on Nov. 21, 2016, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Hot mold steel is a type of alloyed mold steel having iron and different amounts alloy elements, such as carbon, chromium, tungsten, silicon, nickel, molybdenum, manganese, vanadium, and cobalt. A hot mold steel object suitable for molding a material may be prepared from hot mold steel, especially during die casting, extrusion, or die forging. Examples of the mold steel object may include extrusion die, forged mold, die casting mold, press die, or the like, which need to have special mechanical strength properties at high processing temperature and may include ones similar thereto.

An important function of mold steel, especially hot mold steel and a steel object made thereof, is to ensure a sufficient release of heat previously introduced or generated during the process itself when used in a technical process.

The hot mold made of the hot mold steel should have good thermal conductivity and high heat and abrasion resistance with high mechanical stability at high processing temperature. Other important properties of the hot mold steel are excellent hardness and abrasion resistance at high temperature, in addition to sufficient hardness and strength.

The high thermal conductivity of the hot mold steel used to prepare the mold is very important in many applications because it can significantly shorten a cycle time. Since an operation of a hot forming apparatus for performing a hot forming process on a workpiece requires a relatively high cost, a lot of money may be saved by shortening the cycle time. Further, the high thermal conductivity of the hot mold steel is desirable for die casting because the mold used therefor has a much longer lifetime due to the very high temperature durability. The mold steel most often used to manufacture the mold typically has thermal conductivity of about 18 to 24 W/mK at room temperature.

For example, STD61, which is used as the mold steel for the hot forming process, has a high thermal conductivity of less than 28 W/mK. This causes frequent heat check cracking due to an expansion rate difference caused by temperature difference of material parts during a high temperature operation. As a result, the mold lifetime may be decreased and the cooling rate of a produced molded product may not be sufficiently increased, thereby causing deterioration of quality and productivity of a hot stepping product requiring a high cooling rate.

In recent years, with the growing trend of environmentally-friendly and high fuel-efficient vehicles in the automobile industries, the use of lightweight non-ferrous metals has been increasing to achieve weight reduction and a demand for the mold steel for die casting die for forming the lightweight non-ferrous metals has been increasing. However, the price remains very high to create such mold steel for die casting. In addition, in preparing the long life cycle die casting, there is a problem in that the thermal conductivity or durability of the mold steel is not satisfactory.

BRIEF SUMMARY

An object of the present disclosure is to provide hot mold steel capable of preparing long life-cycle die cast having high thermal conductivity and high temperature durability by optimizing compositions of the hot mold steel and preparing conditions thereof.

Other objects and advantages of the present disclosure can be understood by the following description, and become apparent with reference to the embodiments of the present disclosure. Also, it will be appreciated those skilled in the art to which the present disclosure pertains that the objects and advantages of the present disclosure can be realized by the apparatus and methods as claimed and combinations thereof.

In one embodiment, a hot mold steel includes 0.35 to 0.45% by weight of carbon, 0.20 to 0.30% by weight of silicon, 0.30 to 0.40% by weight of manganese, 0.50 to 1.20% by weight of nickel, 1.5 to 2.2% by weight of chromium, 2.0 to 2.6% by weight of molybdenum, 0.0001 to 1.0% by weight of tungsten, more than 0% by weight to 0.4% by weight of titanium, 0.30 to 0.50% by weight of vanadium, 0.0001 to 0.003% by weight of boron, 0.005 to 0.02% by weight of copper, and a balance of the hot mold steel, wherein the balance include an amount of Fe and unavoidable impurities.

One aspect of the present disclosure is using formulas to calculate the preferred amount of each element within the hot mold steel. In one embodiment of the present disclosure, the hot mold steel is represented by Formula (1), which discloses F(1)=F(C)×F(Si)×F(Mn)×F(Cr)×F(Mo)×F(Ni). Each of the sub-formulas are discloses as to determine the hot mold steel's formula. For example, F(C)=0.37-0.39×(0.12̂% by weight of carbon); F(Si)=0.7×% by weight of silicon+1; F(Mn)=3.35×% by weight of manganese+1; F(Cr)=2.16×% by weight of chromium+1; F(Ni)=0.36×% by weight of nickel+1; F(Mo)=3×% by weight of molybdenum+1, wherein F(1) is equal to or greater than 25.

In accordance with one aspect of the present disclosure, hot mold steel contains 0.35 to 0.45 wt % of carbon (C), 0.20 to 0.30 wt % of silicon (Si), 0.30 to 0.40 wt % of manganese (Mn), 0.50 to 1.20 wt % of nickel (Ni), 1.5 to 2.2 wt % of chromium (Cr), 2.0 to 2.6 wt % of molybdenum (Mo), 0.0001 to 1.0 wt % of tungsten (W), more than 0 wt % to 0.40 wt % or less of titanium (Ti), 0.30 to 0.50 wt % of vanadium (V), 0.0001 to 0.003 wt % of boron (B), and 0.005 to 0.02 wt % of copper (Cu) and Fe and unavoidable impurities as the balance, with respect to a total weight.

It may further contain 0.02 to 0.08 wt % of aluminum (Al).

It may further contain 0.005 to 0.06 wt % of nitrogen (N).

It may further contain 0.001 to 0.006 wt % of phosphorus (P) and 0.0001 to 0.002 wt % of sulfur (S).

When each content of carbon, silicon, manganese, chromium, molybdenum, and nickel constituting the hot mold steel is substituted into the following formula 1, the value may be 25 or more.

F(C)×F(Si)×F(Mn)×F(Cr)×F(Mo)×F(Ni)  [Formula 1]

(however, in the above Formula (1),

F(C)=0.37-0.39×(0.12̂ carbon content (%));

F(Si)=0.7×silicon content (%)+1;

F(Mn)=3.35×manganese content (%)+1;

F(Cr)=2.16×chromium content (%)+1;

F(Ni)=0.36×nickel content (%)+1; and

F(Mo)=3×molybdenum content (%)+1)

When each content of carbon, silicon, manganese, chromium, molybdenum, and nickel constituting the hot mold steel is substituted into the following formula 1, the value may be 30 or more.

When each content value of the molybdenum and the tungsten constituting the hot mold steel is substituted into the following Formula (2), the value may be 2 or more and 3 or less.

Molybdenum content (%)+0.5×tungsten content (%)  [Formula 2]

When each content value of the titanium and the vanadium constituting the hot mold steel is substituted into the following Formula (3), the value may be 0.4 or more and 0.5 or less.

Titanium content (%)+vanadium content (%)  [Formula 3]

When each content value of the chromium, the molybdenum and the tungsten constituting the hot mold steel is substituted into the following Formula (4), the value may be 9 or more.

Chromium content (%)+3.3×{molybdenum content (%)+0.5×tungsten content (%)}  [Formula 4]

It may be used for die casting.

In accordance with another aspect of the present disclosure, a method for preparing hot mold steel includes: preparing an steel ingot that contains 0.35 to 0.45 wt % of carbon (C), 0.20 to 0.30 wt % of silicon (Si), 0.30 to 0.40 wt % of manganese (Mn), 0.50 to 1.20 wt % of nickel (Ni), 1.5 to 2.2 wt % of chromium (Cr), 2.0 to 2.6 wt % of molybdenum (Mo), 0.0001 to 1.0 wt % of tungsten (W), more than 0 wt % to 0.40 wt % or less of titanium (Ti), 0.30 to 0.50 wt % of vanadium (V), 0.0001 to 0.003 wt % of boron (B), and 0.005 to 0.02 wt % of copper (Cu) and Fe and unavoidable impurities as the balance, with respect to a total weight; preparing a mold material by forging the steel ingot; quenching the mold material; and performing tempering after the quenching.

The steel ingot may further contain 0.02 to 0.08 wt % of aluminum (Al).

The hot mold steel may further contain 0.005 to 0.06 wt % of nitrogen (N).

The steel ingot may further contain 0.001 to 0.006 wt % of phosphorus (P) and 0.0001 to 0.002 wt % of sulfur (S).

The method may further include: performing an electro-slag remelting (ESR) process prior to forging the steel ingot.

The electro-slag remelting process may be performed under an argon gas atmosphere.

The method may further include: performing preliminary heat treatment on the steel ingot at a temperature of 800 to 1300° C. prior to the forging of the steel ingot.

The forging may be performed at a forging ratio of 5 S or more.

The forging may be performed under a temperature of 850 to 1300° C.

The quenching may be performed under a temperature of 900 to 1030° C.

The tempering may be performed under a temperature of 500 to 630° C.

The method may further include: performing primary tempering at a temperature of 580 to 600° C.; and performing secondary tempering at a temperature of 550 to 590° C.

The method may further include: performing tertiary tempering at a temperature of 610 to 630° C. after performing the secondary tempering.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates results of thermal conductivities of hot mold steels prepared in Example 3 in relation to increase in temperature, in Experimental Example 1;

FIG. 2 is a graph showing values of yield strength with respect to hardness of the hot mold steel prepared in Examples 2, 3 to 5 and 10 and Comparative Examples 2 and 4, in Experimental Example 2;

FIG. 3 is a graph showing values of tensile strength with respect to the hardness of the hot mold steel prepared in Examples 2, 3 to 5 and 10 and Comparative Examples 2 and 4, in Experimental Example 2;

FIG. 4 is a graph showing values of impact energy with respect to the hardness of the hot mold steel prepared in Examples 2, 3 to 5 and 10 and Comparative Examples 2 and 4, in Experimental Example 2;

FIG. 5 is a graph showing measurement results of changes in the thermal conductivities for the hot metal molds prepared in Example 12 and Comparative Examples 7 and 8 depending on each temperature, in Experimental Example 3.

FIG. 6 is a graph showing Charpy U Notch impact energy with respect to the hardness of the hot metal molds prepared in Example 12 and Comparative Examples 7 and 8, in Experimental Example 3;

FIGS. 7(a) and 7(b) each are photographs of surface particle photographs of the hot mold steels prepared in Example 3 and Comparative Example 9, in Experimental Example 5;

FIG. 8 is a graph showing evaluation results of the strength/hardness of the hot mold steel related to quenching temperature as a result of variously controlling the quenching temperatures, in preparing the hot mold steel in the same process in Example 3, in Experimental Example 6;

FIG. 9 is a photograph showing a texture of the finally prepared hot mold steel as a result of variously controlling the quenching temperature and tempering temperature, in preparing the hot mold steel in the same process in Example 3, in Experimental Example 7; and

FIG. 10 is a graph showing evaluation results of the strength/hardness of the hot mold steel depending on the tempering temperature as a result of variously adjusting the tempering temperature, in preparing the hot mold steel in the same process as Examples 1 and 3 depending on different quenching temperatures preparing hot mold steels, in Experimental Example 8.

DETAILED DESCRIPTION

Hereinafter, various embodiments of the present disclosure for creating and using hot mold steel will be described. However, embodiments of the present disclosure may be modified in many different forms and the scope of the invention should not be limited to the embodiments set forth herein. Further, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

Hereinafter, hot mold steel according to an embodiment of the present disclosure will be described.

The hot mold steel according to the present embodiment contains carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), vanadium (V), boron (B) and copper (Cu) as essential elements. Iron (Fe), fine elements, and unavoidable impurities make up a balance of the hot mold steel. As an example, the unavoidable impurities may include phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N), and oxygen (O). Herein, all percentages defined by mass are each equal to those defined by weight. In other words, in describing the amount that an element is within the hot mold steel, the unit of “% by weight” will be used. The compositions of the hot mold steel and the reasons for the numerical limitation will be described below.

Carbon (C)

Carbon (C) is an important element which controls the strength/hardness of steel and also strengthens a solid solution to increase matrix strength and affects ingotability. If carbon suffers from heat treatment, carbide is generated. When the carbon content is less than 0.35% by weight (wt %), hardness and strength are lowered and hardenability is decreased such that uniform cross section hardness may not be obtained. When the carbon content exceeds 0.45 wt %, the hardness is prone to be saturated and an excessive amount of carbide becomes present, which could lead to a deterioration of fatigue strength and impact value. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.35 to 0.45 wt % of carbon.

Silicon (Si)

Silicon (Si) is an important element which controls machinability of steel. Silicon greatly increases thermal conductivity by inhibiting the generation of cementite and promoting carbide generation at high temperature. If the silicon content is less than 0.20 wt %, it is difficult to ensure machinability at or higher than that of STD61, and if the silicon content exceeds 0.30 wt %, the thermal conductivity is greatly decreased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.20 to 0.30 wt % of silicon.

Manganese (Mn)

Manganese is an important element which improves transformation behavior (hardenability) and has the highest effect in ingotability. If the manganese content is less than 0.30 wt %, the transformation point and microstructure refining is decreased as to be insufficient, making it difficult to secure the hardness or the impact value. If the manganese content exceeds 0.40 wt %, the impact value may be decreased so low that the high thermal conductivity cannot be maintained. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.30 to 0.40 wt % of manganese.

Nickel (Ni)

Nickel (Ni) is an element which improves toughness, hardenability, and the stability of the hot mold steel at high temperature. If the nickel content is less than 0.50 wt %, the improvement of the toughness may decrease due to the increase in the hardness and the strength. If the nickel content exceeds 1.20 wt %, austenite may be generated such that the texture may be unstable, a deformation may occur during the use, cutting workability may be decreased, and economical efficiency may be decreased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.50 to 1.20 wt % of nickel.

Chrome (Cr)

Chromium (Cr) is an element which improves hardenability and generates complex carbide to improve hardness, strength, temper softening resistance, and abrasion resistance. If the chromium content is less than 1.5 wt %, the improvement effect of the hardenability is decreased and it becomes difficult to obtain a uniform cross section hardness, and the generation of complex carbides with molybdenum and vanadium is decreased, which leads to a decrease of the temper softening resistance and a decrease in the improvement effect of the strength and the oxidation resistance. On the other hand, if the chromium content exceeds 2.2 wt %, the thermal conductivity is decreased, and the properties of hardness, tensile strength and yield strength may also deteriorate rapidly. Therefore, it is preferable that the hot mold steel of the present disclosure contains 1.5 to 2.2 wt % of chrome.

Molybdenum (Mo)

Molybdenum is an element which improves high temperature hardness and strength by forming carbide (e.g. molybdenum carbide), generating a secondary hardening phenomenon at high temperature during tempering to increase the high temperature strength, and is bonded to the phosphorous in the grain boundary to prevent tempering brittleness during tempering heat treatment. If the molybdenum content is less than 2.0 wt %, the effect of suppressing the temper brittleness is decreased and the secondary hardening phenomenon is lowered, such that the hardness and the strength at high temperature are decreased. Further, if the molybdenum content exceeds 2.6 wt %, the effect of molybdenum is decreased and the economical efficiency is decreased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 2.0 to 2.6 wt % of molybdenum.

Tungsten (W)

Tungsten is an optional element which may be added to increase the strength by precipitation (precipitation hardening) of carbide. If the tungsten content is less than 0.0001 wt %, the effect of increasing the strength becomes small. If the tungsten content exceeds 1.0 wt %, the effect is saturated and cost is considerably increased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.0001 to 1.0 wt % of tungsten.

Titanium (Ti)

Titanium (Ti) is an element which generates strong precipitates, is an element which has low solubility in austenite, and exhibits a microstructure fining effect. In a particular embodiment of the hot mold steel of the present disclosure, it is preferable that the amount of titanium ranges between 0 wt % to 0.40 in order to improve physical properties of hardness and strength. In a particular embodiment, it is preferred that the amount of titanium ranges from 0.10 to 0.40 wt %. In another particular embodiment, it is preferred that the amount of titanium rangers from 0.15 to 0.40 wt %.

Vanadium (V)

Vanadium (V) is an element which is added into iron to become a solid solution therein to increase tensile strength, generate insoluble carbide to increase high-temperature hardness, and increase tempering resistance. In particular, the vanadium (V) exhibits an effect of suppressing austenite particle growth by finely generating stable precipitates at high temperature. If the vanadium content is less than 0.30 wt %, the effect of austenite particle growth suppression may become insignificant. If the vanadium content exceeds 0.50 wt %, a grain refinement phenomenon becomes conspicuous and the hardenability is degraded, such that uniform cross-sectional hardness may not be obtained and economical efficiency may be decreased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.30 to 0.50 wt % of vanadium.

Boron (B)

Boron significantly improves quenching characteristics by segregating grain boundaries, even when an infinitesimal amount of Boron is added. Boron also improves the quenching characteristics of other elements such as manganese, chromium and nickel. It is notable however, that the improvement effect of the quenching characteristics by boron tends to decrease when the amount of carbon is increased. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.0001 to 0.003 wt % of boron.

Copper (Cu)

Copper (Cu) is an element contained in scrap metal. If the hot mold steel's copper content exceeds 0.02 wt %, a surface cracking phenomenon or the like, may occur during a hot forging and decrease the forging performance. Therefore, in one embodiment, the hot mold steel of the present disclosure contains 0.02 wt % or less of copper. In another particular embodiment, the hot mold steel includes 0.005 to 0.02 wt % of copper.

Phosphorous (P)

Phosphorus partially contributes to an increase in strength of the hot mold steel. However, if the phosphorous content exceeds 0.006 wt %, the weldability may deteriorate. Therefore, according to the present disclosure, the phosphorous content of the hot mold steel is preferably 0.006 wt % or less. In a particular embodiment, the hold mold steel includes 0.001 to 0.006 wt % of phosphorous.

Sulfur (S)

Sulfur (S) may be an impurity, and it is preferable that the hot mold steel of the present disclosure contains 0.0001 to 0.002 wt % of sulfur.

Aluminum (Al) and Nitrogen (N)

Aluminum and nitrogen may be impurities introduced during steelmaking and should be decreased as much as possible. However, when boron is added to obtain the grain boundary segregation, if an amount of aluminum and nitrogen are added within a range that does not adversely affect the properties of steel, an aluminum-nitrogen buffering may happen. As described above, the amount of solid boron segregated at the crystal grain boundaries contributes to improvement in the ingotability. Therefore, it is preferable that the hot mold steel of the present disclosure contains 0.02 to 0.08 wt % of aluminum and 0.005 to 0.06 wt % of nitrogen.

In a particular embodiment of the present disclosure, the balance of the hot mold steel is substantially composed of iron (Fe). This means that the steel may contain other infinitesimal amounts of elements, such as unavoidable impurities, as long as it does not hinder the action and effect of the present disclosure.

According to a particular embodiment of the present disclosure, the hot mold steel has high thermal conductivity at high temperatures, and has excellent strength and hardness. This is achieved by controlling, in particular, the chromium content among the compositions to a specific range.

According a particular embodiment of the present disclosure, when each content of carbon, silicon, manganese, chromium, molybdenum, and nickel constituting the mold steel is substituted into the following Formula (1), the total value “F1” is preferably 25 or more.

F(C)×F(Si)×F(Mn)×F(Cr)×F(Mo)×F(Ni)=F1  [Formula (1)]:

The content of each of the respective element used in the Formula (1) is further defined below. It will be appreciated that one way of measuring content (%) is percentage by weight. For example, carbon content (%) may be provided a % by weight of carbon.

F(C)=0.37−0.39×(0.12̂ carbon content (%));

F(Si)=0.7×silicon content (%)+1;

F(Mn)=3.35×manganese content (%)+1;

F(Cr)=2.16×chromium content (%)+1; diameter

F(Ni)=0.36×nickel content (%)+1; and

F(Mo)=3×molybdenum content (%)+1.

According to the present disclosure, the value “F1” obtained from the above Formula (1) (unit: inch) may identify a maximum diameter quenched when the hot mold steel is cooled quickly. The higher the “F1” value, the larger the size of the product that may be made into martensite up to the deep part at the predetermined cooling rate, which is advantageous in the production of the product. Therefore, when the present disclosure considers the thermal conductivity, strength, hardness, or the like of the hot mold steel and also considers the productivity of the product, the value “F1” obtained from the above Formula (1) may preferably may be 25 or more. In other embodiments, the “F1” value may preferably be 26 or more. In yet other embodiments, the “F1” value may preferably be 30 or more.

Further, according to an embodiment of the present disclosure, when the content value of each of the molybdenum and the tungsten is substituted into the following Formula (2), the resulting value “F2” may be 2 or more and 3 or less.

Molybdenum content (%)+0.5×tungsten content (%)=F2  [Formula (2)]:

Formula (2) is a factor for controlling high temperature strength and corrosion resistance. When the value “F2” of the above formula is less than 2 or more than 3, it is difficult to ensure sufficient high temperature strength and corrosion resistance, and therefore the lifetime of the die casting may be shortened when the die casting is manufactured using the hot mold steel.

Further, according an embodiment of the present disclosure, when each content value of the titanium and the vanadium is substituted into the following Formula (3), a “F3” value may be 0.4 or more and 0.5 or less.

Titanium content (%)+vanadium content (%)=F3  [Formula (3)]

Formula (3) provided in the present disclosure is a factor of controlling the high thermal conductivity related to carbide. When the value F3 is less than 0.4 or more than 0.5, it is difficult to secure the sufficient high thermal conductivity, and therefore the quality of the product or the production speed of the product using the die cast prepared from the hot mold steel according to the present disclosure may be affected.

Further, according to an embodiment of the present disclosure, when each content value/% by weight of the chromium, the molybdenum, and the tungsten among the compositions of the hot mold steel is substituted into the following Formula (4), the value “F4” is preferably 9 or more.

Chromium content (%)+3.3×{molybdenum content (%)+0.5×tungsten content (%)}=F4  [Formula (4)]

The above formula (4) provided in the present disclosure is a factor of controlling the corrosion resistance at high temperature. When the value F4 of the Formula (4) is less than 9, it is difficult to ensure sufficient high temperature corrosion resistance. Therefore, the value obtained from the Formula (4) is preferably equal to 9 or more. In another embodiment of the disclosure, the value F4 is preferably equal to 9.5 or more. In yet another embodiment, the value F4 is preferably equal to 10 or more.

According to the present disclosure, the hot mold steel having the above-mentioned compositions may be excellent in providing high temperature durability and high thermal conductivity to decrease the temperature difference in the materials at high temperature, thereby making heat-checking properties excellent. Accordingly, when the hot mold steel according to the present disclosure is used for the die casting, the lifetime of the die casting is long and the cooling rate of the product produced using the die casting is quick, thereby improving the physical properties of the product and shortening the cooling time to improve productivity.

Hereinafter, a method for preparing hot mold steel using the components of the hot mold steel as described above will be described.

In the present disclosure, a steel ingot having the above described compositions may be prepared first. For example, the steel ingot may be prepared by melting metal using an artificial heat source, such as an electric furnace, a vacuum induction furnace, and an air induction furnace, and then effectively removing gases such as oxygen, hydrogen, and nitrogen.

The steel ingot according to the present embodiment may include carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), vanadium (V), and boron (B) as essential elements. The steel ingot further may include iron (Fe), fine elements, and certain impurities (for example, phosphorous (P), sulfur (S), aluminum (Al), nitrogen (N), and oxygen (O)) as a balance of the steel ingot's composition. Preferably, the steel ingot contains 0.35 to 0.45 wt % of carbon (C), 0.20 to 0.30 wt % of silicon (Si), 0.30 to 0.40 wt % of manganese (Mn), 0.50 to 1.20 wt % of nickel (Ni), 1.5 to 2.2 wt % of chromium (Cr), 2.0 to 2.6 wt % of molybdenum (Mo), 0.0001 to 1.0 wt % of tungsten (W), more than 0 wt % to 0.4 wt % or less of titanium (Ti), 0.30 to 0.50 wt % of vanadium (V), 0.0001 to 0.003 wt % of boron (B), and 0.005 to 0.02 wt % of copper (Cu) and Fe and unavoidable impurities as the balance and may further contain 0.02 to 0.08 wt % of aluminum (Al), 0.005 to 0.06 wt % of nitrogen (N), 0.001 to 0.006 wt % of phosphorus (P), and 0.0001 to 0.002 wt % of sulfur (S).

In the present disclosure, the reason for limiting the content of each component in the compositions of the steel ingot is the same as that described in the compositions of the hot mold steel, and therefore a detailed description thereof will be omitted.

In an embodiment of the present disclosure, if the steel ingot is prepared as described above, the steel ingot may be refined by optionally performing an electro-slag remelting (ESR) process.

In an embodiment of the present disclosure, the above-described electro-slag remelting process may be performed under an inert gas atmosphere. In a particular embodiment, the inert gas atmosphere may be an argon gas atmosphere because the material's toughness may be prevented from being weakened due to the formation of nitride by nitrogen dissolved in the air. The above-described electro-slag remelting process may also be performed within a method generally used in the related art, wherein the processing conditions are not limited with the exception of the gas atmosphere.

In an embodiment of the present disclosure, a preliminary heat treatment may be performed on an ingot that was subject to the electro-slag remelting process. The preliminary heat treatment may be performed prior to forging the electro-slag slag remelting ingot into a standard shape.

In an embodiment of the present disclosure, the temperature of the preliminary heat treatment is not particularly limited, but is preferably 800 to 1300° C. If the temperature of the preliminary heat treatment is less than 800° C., the forging may become difficult due to a decrease in temperature during the forging. If the temperature of the preliminary heat treatment exceeds 1300° C., a high temperature embrittlement phenomenon may occur due to overheating.

Further, in an embodiment of the present disclosure, the preliminary heat treatment may include: performing primary heat treatment at a temperature of 1150 to 1300° C. for 15 to 25 hours; and performing secondary heat treatment at a temperature of 1100 to 1200° C. for 8 to 13 hours.

In an embodiment of the present disclosure, after the preliminarily heat treatment is performed, the steel ingot may be forged into a mold material. Specifically, the heat treated steel ingot may be forged at a temperature of 850 to 1300° C. to break a casting texture of the steel ingot. Pores in the steel ingot formed during solidification may be compressed and removed to improve internal quality, and made into a shape of the mold material. In the present disclosure, when the temperature for performing the forging process is less than 850° C., cracks may occur due to the difficulty in deformation during the forging operation. In addition, if the temperature of the forging process exceeds 1300° C., cracking may occur due to high temperature embrittlement phenomenon due to overheating.

Further, in an embodiment of the present disclosure, a forging ratio in the forging process may preferably be 5 S or more. In another embodiment, the forging ratio may be 5 to 10 S. By forging the steel ingot at the forging ratio of 5 S or more, the pores in the steel ingot are compressed to be extinguished, such that the texture of the hot mold steel may be finely formed. However, if the forging ratio is less than 5 S, the texture of the mold steel becomes coarse and therefore the toughness becomes weak. When the mold steel is used in die cast, the quality of the produced product may also deteriorate. On the other hand, if the forging ratio exceeds 10 S, there may be a problem in the limited size of the steel ingot and the working range of the forging press. Therefore, in the present disclosure, it is preferable to perform the forging process at a forging ratio of 5 to 10 S.

In the present disclosure, the mold material obtained by the forging process may be subjected to spheroidizing heat treatment. Due to the forging process, the microstructure and the crystal grain of the mold material become coarse and non-uniform. Therefore, in the present disclosure, the non-uniform crystal grains and the microstructures of the mold material are recrystallized and refined and homogenized by the spheroidizing heat treatment, thereby obtaining satisfactory properties in the quenching and that tempering which are the post processes.

In the present disclosure, the temperature for performing the spheroidizing heat treatment is preferably 650 to 850° C. If the temperature for performing the spheroidizing heat treatment is less than 650° C., the recrystallization and the crystal grains become non-uniform, and the microstructure remains non-uniform even after the spheroidizing heat treatment is performed. If the temperature for performing the spheroidizing heat treatment exceeds 850° C., the crystal grains becomes coarse and it may be difficult to obtain the desired properties by the quenching and tempering processes that are performed later.

In the present disclosure, after the spheroidizing heat treatment as described above is performed, a cooling process may be performed up to a cooling finish temperature of 200 to 300° C. at a cooling rate of 10 to 30° C./hr. The cooling method is not particularly limited, and may rely on any one of oil cooling, air cooling, and water cooling.

In the present disclosure, the quenching process for heat-treating the mold material after the cooling process may be performed. In an embodiment of the present disclosure, a quenching temperature of the quenching process is preferably 900 to 1030° C. In a particular embodiment, the quenching temperature ranges from 940 to 1030° C. If the quenching temperature is less than 900° C., the solid solution effect of added alloying elements is decreased and the homogenization effect of the texture may be decreased. If the quenching temperature exceeds 1030° C., the hardness of the mold material may be suddenly decreased due to the coarsening of particles.

Further, in an embodiment of the present disclosure, after the quenching process, a rapid cooling process may be performed where a cooling finish temperature of 80 to 100° C. at a cooling rate of 0.5° C./s or more may be applied. In a particular embodiment, the cooling rate is between 0.5 to 3.0° C./s and applied using a high-pressure accelerated cooler. This rapid cooling process is performed to improve the strength of the hot mold steel.

In an embodiment of the present disclosure, a process of tempering the rapidly cooled mold steel material as described above may be performed. In the present disclosure, if the temperature for performing the tempering is 500 to 630° C., the brittleness of the steel may be improved, a residual stress may be removed, and the predetermined strength and impact toughness may be obtained due to the formation of fine carbides. When the temperature for performing the tempering is less than 500° C., the residual stress remains because the temperature is low and the effect of improving the toughness of martensite having brittleness is small. If the temperature for performing the tempering exceeds 630° C., the hardness may be suddenly decreased.

Further, in the present disclosure, the rapidly cooled mold steel material is primarily tempered for 3 to 6 hours at a temperature of 580 to 600° C., then secondarily tempered for 3 to 6 hours at a temperature of 550 to 590° C., and then teritarily tempered at a temperature of 610 to 630° C. for 1 to 4 hours.

According to the present disclosure, the primary tempering may remove the residual austenite in the texture of the mold material, form the fine carbide, and improve the strength of the mold material by tempering the martensite.

Further, in the present disclosure, the secondary tempering may form fine carbides in the texture of the mold material, and may improve the strength of the mold material by tempering fresh martensite.

Further, in the present disclosure, the tertiary tempering may precisely control the hardness of the mold steel material.

However, after the primary, secondary and tertiary tempering processes are performed, the cooling of the steel material may be performed up to a temperature of 80° C. or less by using any one of the cooling methods of oil cooling, air cooling and water cooling, thereby obtaining the uniform and fine carbides or the texture of the martensite.

Also, in the present disclosure, it is possible to selectively inspect the tempered mold steel material to determine whether there are defective parts in the mold steel material obtained by the above process. If it is determined that there are defective parts, the defective parts are removed and then the mold steel material may be released.

If the inspection process is completed as described above, the hot mold steel according to the present disclosure may be obtained. The hot mold steel prepared in the present disclosure may be used for die casting that is used for preparing automobile parts.

According to the present disclosure, the hot mold steel having excellent high thermal conductivity and high temperature durability may be prepared finally by preparing the hot mold steel under specific processing conditions and by using the steel ingot having a specific composition. The hot mold steel prepared in the present disclosure may be used for a long period of time and thus is eco-friendly, and the production quality and production speed of the product prepared from the mold steel material may be increased.

Hereinafter, detailed Examples will be described. The following Examples are only an example to help understanding of the present disclosure and the scope of the present disclosure is not limited thereto.

Examples 1 to 12 and Comparative Examples 1 and 8

First, the steel ingot having compositions shown in the following Tables 1 and 2 was prepared and then subjected to the electro-slag remelting process under argon gas atmosphere. The ingot was then forged at the forging ratio of 5.2 S at 1180° C., thereby becoming mold material. Thereafter, the mode material was subjected to spheroidizing heat treatment at 800° C. Thereafter, the quenching, the rapid cooling, and the three steps of tempering processes were performed under the conditions shown in the following Table 3 to prepare the mold material into hot mold steel.

TABLE 1 Weight(%) Ti + Mo + V + And Division C Si Mn Ni P S Cr Mo W 0.5W Ti V 0.5Nb B Al Cu so on Fe Steel of 0.380 0.250 0.400 1.000 0.005 0.001 1.500 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 0.046 0.01 Bal- Example 1 ance Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.000 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 0.055 0.01 Bal- Example 2 ance Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.500 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 0.060 0.01 Bal- Compar- ance ative Example 1 Steel of 0.380 0.250 0.400 1.000 0.005 0.001 3.000 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 0.050 0.01 Bal- Compar- ance ative Example 2 Steel of 0.350 0.250 0.400 1.500 0.005 0.001 1.500 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 0.026 0.01 Bal- Compar- ance ative Example 3 Steel of 0.400 0.250 0.400 1.500 0.005 0.001 1.500 1.500 0.000 1.500 0.100 0.400 0.500 0.0020 Bal- Compar- ance ative Example 4 Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.000 2.000 0.500 2.250 0.100 0.400 0.500 0.0020 0.025 0.01 Bal- Example 3 ance Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.000 2.000 1.000 2.500 0.100 0.400 0.500 0.0020 0.022 0.01 Bal- Example 4 ance Steel of 0.380 0.500 0.400 1.000 0.005 0.001 1.500 2.500 0.000 2.500 0.100 0.400 0.500 0.0020 Bal- Compar- ance ative Example 5 Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.000 2.500 0.000 2.500 0.100 0.400 0.500 0.0000 0.032 0.01 Bal- Example 5 ance Steel of 0.380 0.250 0.400 1.000 0.005 0.001 1.500 2.500 0.000 2.500 0.100 0.300 0.400 0.0020 0.026 0.01 Bal- Example 6 ance Steel of 0.380 0.250 0.400 1.000 0.005 0.001 2.000 2.500 0.000 2.500 0.150 0.300 0.450 0.0020 0.078 0.01 Bal- Example 7 ance Steel of 0.410 0.260 0.400 1.010 0.001 0.002 2.040 2.500 0.001 2.5805 0.090 0.410 0.500 0.001 0.055 0.010 Bal- Example 8 ance Steel of 0.380 1.000 0.400 5.250 1.050 0.850 Bal- Compar- ance ative Example 6 Steel of 0.370 0.380 0.670 0.050 0.010 0.001 5.110 2.670 0.040 2.710 0.001 0.670 0.001 0.040 Nb: Bal- Compar- 0.002 ance ative Co: Example 7 0.0008 Ca: 0.001

TABLE 2 Value F1 of Value F4 of Division Formula (1) Formula (4) Steel of Example 1 26.38 9.75 Steel of Example 2 33.10 10.25 Steel of Comparative Example 1 39.82 10.75 Steel of Comparative Example 2 46.54 11.25 Steel of Comparative Example 3 28.13 9.75 Steel of Comparative Example 4 20.04 6.45 Steel of Example 3 27.26 9.46 Steel of Example 4 27.36 10.25 Steel of Comparative Example 5 30.31 9.75 Steel of Example 5 33.10 10.25 Steel of Example 6 26.38 9.75 Steel of Example 7 33.10 10.25 Steel of Example 8 36.80 10.56 Steel of Comparative Example 6 39.88 5.25 Steel of Comparative Example 7 87.09 14.053

TABLE 3 Primary Secondary Tertiary Division Steel ingot Quenching Cooling tempering tempering tempering Example 1 Steel of Example 1 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 2 Steel of Example 1 1030° C./1 hr 6Bar/Nitrogen pressure 590° C./2 hr 640° C./2 hr 650° C./2 hr Example 3 Steel of Example 2 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 4 Steel of Example 2 1030° C./2.5 hr Oil cooling 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 5 Steel of Example 2 1030° C./1 hr 6Bar/Nitrogen pressure 590° C./2 hr 640° C./2 hr 650° C./2 hr Comparative Steel of Comparative 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr Example 1 Example 1 Comparative Steel of Comparative 1030° C./1 hr 6Bar/Nitrogen pressure 590° C./2 hr 640° C./2 hr 650° C./2 hr Example 2 Example 1 Comparative Steel of Comparative 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr Example 3 Example 2 Comparative Steel of Comparative 1030° C./1 hr 6Bar/Nitrogen pressure 590° C./2 hr 640° C./2 hr 650° C./2 hr Example 4 Example 2 Comparative Steel of Comparative 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 5 Example 3 Comparative Steel of Comparative 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 6 Example 4 Example 6 Steel of Example 3 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 7 Steel of Example 4 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 8 Steel of Example 5 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 9 Steel of Example 6 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 10 Steel of Example 7 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./2 hr Example 11 Steel of Example 7 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 12 Steel of Example 8 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Comparative Steel of Comparative 1030° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 7 Example 6 Comparative Steel of Comparative 1020° C./2.5 hr 8Bar/Nitrogen pressure 600° C./5 hr 570° C./4 hr 630° C./4 hr Example 8 Example 7

[Experimental Example 1] Evaluation of Thermal Conductivity

Thermal conductivities of the hot mold steel prepared in the above Example 3 were measured as compared to each temperature and the results were shown in the following Table 4 and FIG. 1.

TABLE 4 Temperature (° C.) Thermal conductivity (W/mK) 25 30.73 100 35.142 200 35.982 300 35.161 400 34.384 500 34.03 600 32.502 650 32.188 700 31.611

As shown in the above Table 4 and FIG. 1, the hot mold steel prepared according to the present disclosure exhibited the thermal conductivity of 30 W/mK or more at room temperature or higher, the thermal conductivity of about 35 W/mK or more at a high temperature of 100° C. or higher, and a thermal conductivity of 31 W/mK or more even at a very high temperature of 600 to 700° C.

As a result, it may be seen that the hot mold steel according to the present disclosure is excellent in high thermal conductivity.

[Experimental Example 2] Evaluation of Physical Properties

To visualize the change in strength in view of the change in the contents of chromium and titanium, the values of the yield strength and the tensile strength as a function of the hardness of the hot mold steel prepared in Examples 2, 3 to 5 and 10, and Comparative Examples 2 and 4 are shown by the graphs in FIGS. 2 and 3. The values of the impact energy with respect to the hardness is shown by the graph in FIG. 4.

As shown in FIGS. 2 and 3, Examples 2, 3 to 5, and 10, when the content of chromium falls within the range of the present disclosure, the hardness, the yield strength, and the tensile strength are very high. By contrast, in Comparative Examples 2 and 4, the content of chromium is out of the range of the present disclosure, which results in that the hardness and the yield strength have significantly low value. In addition, it may be seen that the hardness and the yield strength of Example 10 are higher than those of Examples 2 and 3 to 5.

As shown in FIG. 4, Examples 2, 3 to 5 and 10 in which the content of chromium was in the range of the present disclosure may show that, the impact energy was extremely low, while Comparative Examples 2 and 4 in which the content of chromium was out of the range of the present disclosure may show that the impact energy has a very high value. In addition, it may be seen that the impact energy of Example 10 are even lower than that of Examples 2 and 3 to 5.

[Experimental Example 3] Evaluation of Physical Properties

The change in the thermal conductivity of the hot mold steel prepared in Example 12 and Comparative Examples 7 and 8 as related to the temperature is shown in FIG. 5; the results of performing the heat-check is shown in the following Table 5, and the results of evaluating the toughness is shown in FIG. 6.

The heat-check evaluation conditions are as follows: the heating was performed for 13 seconds at 700° C. and the cooling was performed for 12 seconds. The heating and cooling were repeated 1,000 times to measure an average crack length (mm) and a maximum crack length (mm). In addition, the toughness evaluation was evaluated at U-notch impact toughness of 20 J/cm² or more according to the standard NADCA.

TABLE 5 Average crack Maximum crack Division length (mm) length (mm) Comparative Example 7 0.46 3.78 Comparative Example 8 0.12 0.5 Example 12 0.09 0.73

As shown in FIGS. 5 and 6, it may be seen that the hot mold steel of Example 12 has even more excellent properties of the thermal conductivity and the impact toughness than those of Comparative Examples 7 and 8.

Further, as may be seen from the above Table 5, it may be seen that the heat-check property of Example 12 is even more excellent than that of Comparative Examples 7, and is equivalent to or more than that of Comparative Example 8.

[Experimental Example 4] Evaluation of Electro-Slat Remelting Conditions

The electro-slag remelting process was performed on the steel ingot of the above Example 1 under the conditions of air (20 tons of injection) or argon gas (100 kg of injection), respectively. The content of hydrogen, oxygen and nitrogen in the steel ingot that is obtained in the melted material of the steel ingot and the steel ingot obtained by performing the electro-slag remelting process was evaluated, and the results are shown in the following Table 6.

TABLE 6 Composition of steel ingot Gas condition during ESR process H (ppm) O (ppm) N (ppm) Melted state (before gas injection) 1.5 69 30 Air injection 1.4 22 115 Argon gas injection 0.6 17 58

As shown in Table 6, it may be seen that to perform the electro-slag remelting process under the argon gas atmosphere to prepare the hot mold steel, the nitrogen content is slightly increased compared to the dissolved state. However, when air is injected, nitride is formed in a substantial amount of nitrogen dissolved from the air.

[Experimental Example 5] Evaluation of Forging Conditions

In order to compare the difference according to the forging ratio, the hot mold steel was prepared in Example 3, and Comparative Example 9 prepared the hot mold steel in the same process as in Example 3 except Comparative Example 9 has a forging ratio of 3.2 S. The resulting photograph of the surface particles of the hot mold steel of the above Example 3 and the Comparative Example 9 were shown in FIGS. 7(a) and 7(b).

Specifically, as shown in FIG. 7(a), it may be seen that the hot mold steel of the above Example 3 has a dense structure with a particle size of ASTM #7 or more. By contrast, the hot mold steel of the above Comparative Example 9 has coarsened particles with a particle size of about ASTM #2.5 as shown in FIG. 7(b).

[Experimental Example 6] Evaluation of Quenching Temperature Conditions

In order to evaluate the change in strength/hardness according to the quenching temperature condition, the hot mold steel was prepared by the same process as in the above Example 3, and the quenching temperature was controlled to 940° C., 970° C., 1000° C., 1030° C. and 1060° C. The strength of the hot mold steel finally prepared according to the quenching temperature was evaluated and the results were shown in FIG. 8.

As may be seen from FIG. 8, as the quenching temperature increases to 940° C., 970° C. and 1000° C., the strength/hardness of the hot mold steel was also increased and the most excellent strength was observed at 1030° C. while the strength suddenly decreased at 1060° C.

[Experimental Example 7] Evaluation of Quenching Temperature and Tempering Temperature Conditions

In order to evaluate the strength change according to the quenching temperature and the tempering temperature condition, the hot mold steel was prepared by the same process as in the above Example 3, in which the quenching temperature was controlled to 1000° C., 1020° C., and 1040° C., the tempering was performed in one step, and the performance temperatures was controlled to 400° C., 550° C., and 625° C., respectively. A photograph of the texture of the hot mold steel finally prepared according to the quenching temperature and the tempering temperature was shown in FIG. 9.

As shown in FIG. 9, it may be seen that when the quenching is performed at 1000° C. and 1020° C. and the tempering is performed at 550° C., the texture particles of the hot mold steel are fine and the hot mold steel has a stable structure. However, it may be seen that when the quenching is performed at 1040° C., even if the tempering is performed at 550° C., the particles of hot mold steel are coarsened and the texture is irregular.

Therefore, according to the above Experimental Examples 6 and 7, it may be seen that if the quenching is performed at 940 to 1030° C., the texture of the hot mold steel gets dense to increase the strength and the toughness.

[Experimental Example 8] Evaluation of Tempering Temperature Conditions

In order to evaluate the change in strength according to the tempering temperature conditions, the hot mold steel was prepared in the same process as in the above Examples 1 and 3, in which the tempering was performed in one step and the performance temperature was controlled to 0° C., 400° C., 500° C., 550° C., 565° C., 580° C., 600° C., 625° C., and 650° C. The strength of the hot mold steel finally prepared according to the tempering temperature was evaluated and the results were shown in FIG. 10.

As may be seen from FIG. 10, when the tempering is performed at a temperature of 500 to 600° C., the hardness/strength is maximized. However, it may be seen that when the performance temperature of the tempering exceeds 600° C., the hardness is drastically decreased.

According to an embodiment of the present disclosure, the hot mold steel may be excellent in the high thermal conductivity to decrease the temperature difference in the materials at the high temperature, thereby making the heat-checking properties excellent. Accordingly, when the hot mold steel of the present disclosure is used for the die casting, the cooling rate of the product produced using the die casting is quick, thereby improving the physical properties of the produced product and shortening the cooling time to improve the productivity.

Furthermore, the hot mold steel of the present disclosure have excellent high temperature durability, such that the die cast product produced using the hot mold steel may have characteristics of a long life cycle.

While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the spirit and scope of the present disclosure as defined by the appended claims. 

What is claimed is:
 1. A hot mold steel, comprising: 0.35 to 0.45% by weight of carbon; 0.20 to 0.30% by weight of silicon; 0.30 to 0.40% by weight of manganese; 0.50 to 1.20% by weight of nickel; 1.5 to 2.2% by weight of chromium; 2.0 to 2.6% by weight of molybdenum; 0.0001 to 1.0% by weight of tungsten; 0 to 0.40% by weight of titanium; 0.30 to 0.50% by weight of vanadium; 0.0001 to 0.003% by weight of boron; 0.005 to 0.02% by weight of copper; and iron.
 2. The hot mold steel of claim 1, further comprising 0.02 to 0.08% by weight of aluminum.
 3. The hot mold steel of claim 1, further comprising 0.005 to 0.06% by weight of nitrogen.
 4. The hot mold steel of claim 1, further comprising 0.001 to 0.006% by weight of phosphorus and 0.0001 to 0.002% by weight of sulfur.
 5. The hot mold steel of claim 1, wherein: F(1)=F(C)×F(Si)×F(Mn)×F(Cr)×F(Mo)×F(Ni) F(C)=0.37−0.39×(0.12̂% by weight of carbon) F(Si)=0.7×% by weight of silicon+1 F(Mn)=3.35×% by weight of manganese+1 F(Cr)=2.16×% by weight of chromium+1 F(Ni)=0.36×% by weight of nickel+1 F(Mo)=3×% by weight of molybdenum+1 and F(1) is equal to or greater than
 25. 6. The hot mold steel of claim 5, wherein F(1) is equal to or greater than
 30. 7. The hot mold steel of claim 1, wherein F(2)=% by weight of molybdenum+0.5×% by weight of tungsten and F(2) is in the range of 2 to
 3. 8. The hot mold steel of claim 1, wherein F(3)=% by weight of titanium+% by weight of vanadium and F(3) is in the range of 0.4 to 0.5.
 9. The hot mold steel of claim 1, wherein F(4)=% by weight of chromium+3.3×(% by weight of molybdenum+0.5×% by weight of tungsten) and F(4) is equal to 9 or more.
 10. A method of die casting, comprising die casting with the hot mold steel of claim
 1. 11. A method for preparing hot mold steel, comprising: preparing an steel ingot that contains 0.35 to 0.45% by weight of carbon, 0.20 to 0.30% by weight of silicon, 0.30 to 0.40% by weight of manganese, 0.50 to 1.20% by weight of nickel, 1.5 to 2.2% by weight of chromium, 2.0 to 2.6% by weight of molybdenum, 0.0001 to 1.0% by weight of tungsten, 0.30 to 0.50% by weight of vanadium, 0.0001 to 0.003% by weight of boron, and 0.005 to 0.02% by weight of copper, and iron; preparing a mold material by forging the steel ingot; quenching the mold material; and tempering the mold material after the quenching.
 12. The method of claim 11, further comprising performing an electro-slag remelting (ESR) process prior to forging the steel ingot.
 13. The method of claim 12, wherein the ESR process is performed under an argon gas atmosphere.
 14. The method of claim 11, further comprising performing a preliminary heat treatment on the steel ingot at a temperature of 800 to 1300° C. prior to the forging of the steel ingot.
 15. The method of claim 11, wherein the forging is performed at a forging ratio of 5 S or more.
 16. The method of claim 11, wherein the forging is performed at a temperature of 850 to 1300° C.
 17. The method of claim 11, wherein the quenching is performed at a temperature of 900 to 1030° C.
 18. The method of claim 11, wherein the tempering is performed at a temperature of 500 to 630° C.
 19. The method of claim 11, wherein the tempering includes performing primary tempering at a primary temperature of 580 to 600° C., and performing secondary tempering at a secondary temperature of 550 to 590° C.
 20. The method of claim 19, further comprising: performing tertiary tempering at a tertiary temperature of 610 to 630° C. after performing the secondary tempering. 